This research will be done primarily in Russia as an extension of NIH Grant D/GM-56729-02. The DNA structure of Drosophila melanogaster telomeres has been worked out in sufficient detail, but very little is known about its chromatin structure and the telomere binding proteins. We propose that there are two distinct protein complexes located at Drosophila telomeres: an end-binding complex required for "capping," and another protein complex associated with the subtelomeric repetitive (TAS) region, which is involved in gene silencing. While the parent grant R0l GM56729 (05/01/1998-04/30/03) studies the latter, this FIRCA supplement will focus on the characterization of the end-binding complex, which is located at the termini of Drosophila chromosomes to prevent telomeric fusions and protect the end from being recognized as double strand DNA break by cell cycle checkpoints. No components of this complex have yet been identified, but our preliminary work has, for the first time, provided genetic evidence for the existence of such a terminal capping structure. This newly discovered terminal deficiency effect (called TDE) will be used to characterize protein components of the end-binding complex. To this end, we will investigate if the chromatin structure of a DNA region is altered when it is located at the end of a terminally deleted chromosome. This will provide a "footprint" of the terminal capping complex, and determine the size of the affected region. Effects of TDE modifiers on the terminal chromatin structure will also be studied. Molecular and genetic approaches will be used to investigate a possible role of two candidate proteins (HP1 and Ku) in TDE, and genetic screens will be performed to identify genes and their protein products that may participate in the formation of the telomeric protein complex. Results from these experiments will complement and extend studies supported in the parent grant and greatly increase our understanding of the complex nucleo-protein structure at the chromosome ends.